Fiber optic rotary joints exist in a myriad of different forms and structures. In many cases, such joints have a rotor mounted for rotational movement relative to a stator. One or more optical signals can be transmitted across the interface between the rotor and stator (i.e., from the rotor to the stator and/or vice versa).
Contactless off-axis fiber optic rotary joints have been developed, such as disclosed in U.S. Pat. No. 4,525,025 A, which is assigned to the assignee of the present invention. The '025 patent discloses a fiber optic rotary joint that transmits a pulsed optical signal across a rotary interface. This device includes an annular reflective wall formed on a stator, and an optical fiber also mounted on the stator and having one end arranged tangentially proximate the annular reflective wall. A signal emitted by one of a plurality of rotor-mounted optical fibers will be transmitted across the annular interface between the rotor and stator, will be reflected along the annular reflective wall, and will be received by a respective one of the stator-mounted optical fibers.
Actual joints constructed in a manner generally similar to that disclosed in the '025 patent have been limited to rotor diameters of about ten to twelve inches, and data transmission rates of 50 megabits/sec (“Mbps”), due to various propagation delays that cause bit pulse-width distortion. There is a need for joints having rotor diameters of 101.6-127.0 centimeters [i.e., 40-50 inches] using pulsed optical signals having data transfer rates of 1-3 gigabits/sec (“Gbps”) or more. To meet these requirements, two criteria must be met. First, optical variations with rotation must be minimized. Second, propagation delays must be controlled to minimize their effect on bit pulse-width distortion.
U.S. Pat. No. 5,336,897 A discloses an optical data transmission apparatus which is used to transmit a signal between the rotating and fixed portions of an X-ray CT scanner. The apparatus includes light-emitting elements arranged on a side plane of the rotating portion, which side plane is perpendicular to the rotational axis of such portion. The light-emitting elements are uniformly driven according to transmission data to emit lights in a direction parallel to the rotational axis. A light-receiving element is disposed on the side plane of the fixed portion which faces the light-emitting elements. The interval between the light-emitting elements is set so that the illumination areas formed by the various light-emitting elements partly overlap each other on the light-receiving element. Therefore, the light-receiving element always receives light from one or two of the light-emitting elements. During rotation of the rotating portion, data can be continuously transmitted from all of the light-emitting elements to the various light-receiving elements.
U.S. Pat. No. 5,991,478 A and U.S. Pat. No. 6,104,849 A disclose FORJs having a waveguide on the stator. Unidirectional and bidirectional FORJs are disclosed for transmitting at least one optical signal across a rotary interface. The FORJs include a stator having a waveguide. The rotor is rotatable through 360°, and is concentric with the stator. Light transmitters are positioned on a first circumference on one of the stator and rotor. Each of the transmitters emits an optical signal. Light receivers are positioned on a second circumference of the other of the rotor and stator. Each of the transmitted optical signals is emitted tangentially into the waveguide, and is reflected in short chordal lengths therealong. Each optical signal is received by at least one of the second plurality of light receivers at any relative angular position between the rotor and the stator. The number of light receivers is greater than the number of light transmitters. Certain receivers do not receive an optical signal during a portion of the 360° revolution of the rotor.
U.S. Pat. No. 6,385,367 B1 discloses FORJs comprised of multiple segmented circumferentially-spaced waveguides located on the stator. Spaces between the waveguides are non-reflective. Each waveguide has an optical pickup. A plurality of optical transmitters are located on the rotor. In the preferred embodiment, there are sixteen transmitters, with eight transmitters transmitting at any given time and eight transmitters turned off at that same time. This reference also teaches the use of a switch for routing each input data stream to the appropriate transmitter that transmits a corresponding optical signal to a predetermined waveguide segment for that particular angular position of the rotor relative to the stator such that each transmitter will transmit an individual optical signal to its associated waveguide at that particular angular position.
U.S. Pat. No. 6,453,088 B1 discloses segmented waveguides for large-diameter FORJs. The waveguides are mounted to the existing stator surface. Each waveguide is capable of receiving signals from the rotor. The FORJ includes a rotor and an existing stator surface. The rotor is rotatable through 360°, and is concentric with the existing stator surface. The rotor has one of a plurality of light transmitters and light receivers connected to a first circumference of the rotor. The waveguides include a reflective waveguide surface shaped to match a portion of the existing stator surface. At least one waveguide support holds the reflective waveguide surface and is connected to the existing stator. At least one of a light transmitter or light receiver is optically coupled to a reflective waveguide surface.
U.S. Pat. No. 6,907,161 B2 discloses FORJs that eliminate the lens/prism assemblies and the multiple pick-up fibers that must be multiply-lensed to a detector to get sufficient signal strength for the system to work. The FORJ also compensates for some of the rapid rise and fall time of certain system components. A single pick-up, either a fiber or a photodiode, is placed at the end of a waveguide. A lens or lens system is used to focus a single optical signal onto the fiber face or the photodiode active area. Various light injection techniques, such as fibers, fiber/lens assemblies, lensed VCELs, lasers, LEDs and the like, can be utilized because of the location in the system.
U.S. Pat. No. 6,980,714 B2 discloses FORJs and associated reflector assemblies for supporting optical communications between a rotor and a stator. The FORJs include at least one optical source carried by the rotor or the stator for transmitting optical signals. The FORJ also includes a reflector mounted on the other of the rotor and stator for reflecting the optical signals, and a receiver for receiving the optical signals following their reflection. The reflector is generally shaped and positioned such that the path lengths along which the optical signals propagate from the optical source(s) to the receivers are equal, regardless of the rotational position of the rotor relative to the stator. The reflector may have a reflective surface shaped to define a portion of an ellipse and/or a reflective surface shaped to define a portion of a hyperbola.
International Pub. No. WO 2007/130016 A1 discloses optical rotary joints for enabling optical communication between a rotor and a stator, improved methods of mounting such optical rotary joints on supporting structures such that the rotor and stator remain properly aligned, and improved optical reflector assemblies for use in such optical rotary joints. The improved optical rotary joints enable optical communication between a rotor and a stator. The rotor has a longitudinal axis, and includes at least one optical source mounted on one of said rotor and stator for transmitting an optical signal in a radial direction with respect to the longitudinal axis, and at least one first reflector mounted on the other of the rotor and stator for reflecting the optical signal transmitted from the source. The first reflector includes a concave first reflective surface. A line in a plane taken through the first reflective surface is configured as a portion of an ellipse having first and second focal points. The first focal point is positioned substantially coincident with the rotor axis. A second reflector, having a second reflective surface configured as a portion of a cone, is positioned at the second focal point of the elliptical surface for receiving light reflected from the first reflective surface, and reflects light in a different direction as a function of the apex angle of the second reflective surface. A receiver is arranged to receive light reflected by the second reflective surface.
Finally, U.S. Pat. No. 7,158,700 B2 discloses fiber-optic transceivers in which a light source and a photodiode are arranged in aligned spaced relation to the proximal end of an optical fiber. The light source is arranged to emit light into the fiber, and the photodiode is arranged to receive light from the fiber.
In CT scanner applications, in which the axis of rotation of a rotor is sometimes physically occupied by a patient, off-axis rotary joints are generally employed to transmit signals between the rotor and stator. Such off-axis rotary joints generally include one or more light sources for emitting optical signals, and arcuate reflectors having channel-shaped transverse cross-sections that receive such transmitted signals and direct such received signals to respective light receivers. The optical sources are spaced circumferentially about one of the rotor and stator, while the reflectors and receivers are spaced circumferentially about the other of the rotor and stator. The optical sources may include one or more common light sources. The optical signals from these light sources may be directed, as by optical fibers, to the periphery of the associated one of the rotor and stator. Alternatively, the optical sources may be separate emitting elements mounted about such periphery. For example, the optical sources may be disposed circumferentially about the rotor, while the multiple reflectors and receivers may be disposed circumferentially about the stator, thereby supporting optical communications from the rotor to the stator. In most cases, the path of optical data transmission across the rotary joint (i.e., between the rotor and stator) is in a radial direction with respect to the rotor axis. In other words, if light is transmitted from the rotor to the stator, the light is seen as coming from the rotor axis, regardless of the physical location of the light source(s).
In operation, each of the light sources may possibly transmit the same optical signal. These signals may be transmitted across the rotary interface, and may be received by one or more of the reflectors and be directed to the associated receivers, depending upon the angular position of the rotor relative to the stator. In other embodiments, different optical signals may be transmitted from different light sources, or may be multiplexed if coming from the same source.
While generally effective for permitting optical communication between a rotor and a stator, some conventional off-axis rotary joints that employ such arcuate reflectors with channel-shaped cross-sections suffer from certain shortcomings, especially at higher data transmission rates. These problems may include: (a) the broadening of superimposed pulse widths due to different-length light transmission paths, and (b) a greater number of light sources must be used when transmitting signals into the entrance end of an optical fiber than when such signals are incident directly upon a photodetector, as discussed infra. In addition, some signal collection arrangements may have variable optical path lengths that practically limit the design to data transmission rates of about 2.5 Gbps.
For example, in conventional off-axis rotary joints, the optical signals may travel along different-length paths between the various sources and the respective receivers, thereby introducing time delays in the various received optical signals, when superimposed. A particular receiver might receive signals from two circumferentially-adjacent optical sources. If the same optical signal is simultaneously emitted by the two adjacent sources, but such signals travel different distances to reach the receiver, the signals will be received at different times. Accordingly, the two signals will be out-of-phase, and the pulse width of the superimposed signals, as seen by the receiver, will be effectively broadened. To support communication at the desired high data rates, conventional off-axis rotary joints have been specifically designed to have less spacing between the optical sources and the receivers so as to minimize the path lengths of signal travel. Even so, it is difficult to support error-free data transmission at a data rate above 1.25 Gbps, where the signals travel along different-length paths.
The aggregate disclosures of each of the foregoing patents are hereby incorporated by reference.
Accordingly, it would be generally desirable to provide improved low-cost FORJs that are capable of high data rate transmission.